1. Field of the Invention
The present invention relates to a backlight module and, particularly, to an edge-lighting type backlight module for use in, for example, a liquid crystal display (LCD).
2. Discussion of the Related Art
In a liquid crystal display device, liquid crystal is a substance that does not itself radiate light. Instead, the liquid crystal relies on receiving light from a light source to thereby display images and data. In the case of a typical liquid crystal display device, a backlight module powered by electricity supplies the needed light.
FIG. 8 (Prior art) represents a conventional edge-lighting type backlight module 80. The backlight module 80 includes a plurality of light emitting diodes (LEDs) 81, a light guide plate 83, a diffusion sheet 84, and a prism sheet 85. The light guide plate 83 includes an incident surface 832 and an emitting surface 834 adjacent the incident surface 832. The LEDs 81 are positioned adjacent the incident surface 832 of the light guide plate 83. The diffusion sheet 84 is located above the light guide plate 83 and is configured for uniformly diffusing the emitted light. The prism sheet 85 is positioned above the diffusion sheet 84 and is configured for collimating the emitted light, thereby improving the brightness of light illumination. However, a plurality of dark areas are generally unavoidably formed adjacent the incident surface 832.
Referring to FIG. 9, another conventional backlight module 90 is shown. The backlight module 90 includes a plurality of LEDs 91 and a light guide plate 93. The light guide plate 93 includes an incident surface 932 facing the LEDs 91, an emitting surface 934 adjoining the incident surface 932, and a plurality of light diffusing portions 936 defined in the incident surface 932, spatially corresponding to the LEDs 91. The light diffusing portions 936 has a plurality of V-shaped grooves arranged regularly and periodically in a direction perpendicular to the emitting surface. Light beams incident on the light diffusing portions 936 are scattered. The diffusing portions 936 can thereby reduce the area of dark areas formed adjacent the incident surface 932.
FIG. 10 shows that a light beam is refracted into the light guide plate 93 through the incident surface 932 having a plurality of diffusing portion 936 of FIG. 9. According to the Fresnel formula of reflection and deflection, a deflection angle β can be defined by the following equation:
      β    =          90      -              α        /        2            -              arcsin        (                              sin            ⁡                          (                              90                -                                  α                  /                  2                                            )                                n                )              ,wherein α is the vertex angle of V-shaped grooves of the diffusing portion 936, and n is a refractive index of the light guide plate 90. The deflection angle β cannot be greater than or even equal to 90 degrees, according to the equation. For example, if the light guide plate 90 is formed of polymethyl methacrylate (PMMA), the largest deflection angle β is generally smaller than 50 degrees. Therefore, the diffusing portions 936 cannot eliminate dark areas formed adjacent the incident surface 932 completely. In addition, a part of light beams can be reflected at the incident surface 932 of the light guide plate 93, thus a utilization efficiency of light energy of the backlight module 90 is decreased.
Referring to FIG. 11, a still another conventional backlight module 100 is shown. The backlight module 100 is similar to the backlight module 90, except that the light diffusing portions 136 are different from the diffusing portion 936 of the backlight module 90. The backlight module 100 includes a light guide plate 130 having an incident surface 132, an emitting surface 134, and a plurality of light diffusing portions 136. The light diffusing portions 136 are, particularly, a plurality of grooves defined in the incident surface 132. The diffusing portions 136 can also reduce the area of the dark areas formed adjacent the incident surface 132.
Referring to FIG. 12, a yet another conventional backlight module 120 is shown. The backlight module 120 includes a plurality of LEDs 141, a light guide plate 140, and a reflector 145. The light guide plate 140 includes an incident surface 142, which faces the LEDs 141, and an emitting surface 144 adjoining the incident surface 142. The reflector 145 has a plurality of curved sections, each partly surrounding the respective LED 141. Each of the LEDs 141 has a luminescent surface 143 that faces the adjacent curved section of the reflector 145. Light beams, emitted from the LEDs 141, are redirected by the reflector 145 and enter into the light guide plate 140 through the incident surface 142 thereof. However, part of the light beams are blocked by the respective LEDs 141, thereby preventing that part of the light beams from reaching the incident surface 142 adjacent to the respective LEDs 141. As a result, a plurality of dark areas is formed in the light guide plate 140, adjacent the incident surface 142.
What is needed, therefore, is a backlight module which can completely eliminate the dark areas formed adjacent the incident surface of the light guide plate.